Typically, the surfaces of glass fiber substrates are coated with a sizing composition during the forming process to protect the glass fibers from interfilament abrasion. Such sizing compositions can include as components film-formers, lubricants, coupling agents, emulsifiers, antioxidants, ultraviolet light stabilizers, colorants, antistatic agents and water, to name a few.
Conventional sizing compositions, however, do not impart to the glass fibers sufficient flexural strength, stiffness, tensile strength, acid resistance, outgassing and compressive strength when such glass fibers are used in applications where such properties are important, for example, reinforcement for optical fibers, brushes, belts, hoses, filter felt for the paper industry, carpet backing and braiding for electrical wiring insulation.
In woven and nonwoven fabrics, for example, attempts to solve the problem of providing a glass fiber with sufficient flexibility are described in U.S. Pat. Nos. 4,762,750 and 4,762,751. These patents teach a secondary coating having elastomeric curable polyurethanes, crosslinking agents, softeners and water. The fibers treated with these compositions do not possess the desired amount of stiffness for some of the applications described above, such as optical fiber cable reinforcement.
U.S. Pat. No. 5,182,784 discloses an aqueous coating composition for glass fibers consisting essentially of a thermoplastic polyurethane latex, an acrylic latex, and either a second acrylic latex or paraffin wax. This coating is disclosed as reducing water wicking by glass fibers in applications such as optical fiber cable reinforcement.
Accordingly, a need exists for less expensive coatings for glass fibers which are compatible with a range of polymers and can be used in a number of applications in which relative stiffness, flexural strength, tensile strength, acid and caustic resistance, abrasion resistance, outgassing and compressive strength are needed.